1. Field of the Invention
This invention relates to a surface-condition inspection apparatus, and more particularly, to a surface-condition inspection apparatus which is suitable for detecting pattern defects or foreign particles, such as dust particles or the like, present on a substrate, such as a reticle, a photomask or the like, used in the production of semiconductor devices.
2. Description of the Prior Art
In general, in the IC (integrated circuit) production process, circuit patterns to be exposed are formed on a substrate, such as a reticle, a photomask or the like, and are transferred onto the surface of a wafer coated with a resist using a semiconductor printing apparatus (a stepper or a mask aligner).
If foreign particles, such as dust particles or the like, are present on the surface of the substrate at the time of circuit pattern transfer, the foreign particles are also transferred, causing a decrease in the yield of the IC production.
In particular, when circuit patterns are printed onto the surface of a wafer by the step-and-repeat method using a reticle, one foreign particle present on the surface of the reticle is printed onto the entire surface of the wafer, causing a great decrease in the yield of the IC production. Accordingly, the ability to detect the presence of a foreign particle on a substrate in the IC production process is indispensable, and various kinds of inspection apparatuses have been proposed for that purpose.
FIG. 15 shows an example of such a conventional apparatus. This apparatus has a feature in that one light beam is simultaneously projected onto two surfaces to be inspected (a pattern surface and a blank surface) of a reticle, whereby time for inspection is shortened. That is, an incident light beam 3 passing through an f-8 lens 2, serving as a lens for projecting light, is divided into two light beams by a half-mirror 4. The resulting two light beams are focused onto points P and Q on the respective surfaces of the reticle by reflecting mirrors 5 and 10 provided above and below the reticle 1, respectively. Usually, the surface of the reticle having circuit patterns (the pattern surface) faces downward, and the other surface which is blank (the blank surface) faces upward. When inspecting for circuit defects, only the pattern surface needs to be inspected. However, when inspecting foreign particles, both the pattern surface and the blank surface must be inspected. A rotating element (a polygonal mirror, not shown) is provided in front of the projection lens 2, and performs scanning of the light beam in a direction orthogonal to the plane of FIG. 15. The upper and lower light beams thereby scan the surfaces of the reticle 1 in the direction orthogonal to the plane of FIG. 15. The reticle 1 is moved in the direction S.sub.1 .rarw.S.sub.2 in synchronization with the scanning of the light beams, whereby the entire surfaces of the reticle I are inspected. Thus, the pattern surface and the blank surface of the reticle 1 are subjected to two-dimensional scanning (raster scanning). Scattered light reflected from the incident point P on the reticle 1 is imaged onto a field stop 7a by the function of a light-receiving lens 6a. The field stop 7a has the function of guiding only necessary signal light to a fiber 8a and a photomultiplier 9a which follow, and shielding other unnecessary light. According to the above-described configuration, an upper light-receiving system 30 is formed, which enables inspection for the presence of foreign particles or the like using the output of the photomultiplier 9a. A light-receiving system 31 for scattered light emanating from the incident point Q has the same configuration (a light-receiving lens 6b, a field stop 7b, a fiber 8b and a photomultiplier 9b) as the above-described configuration.
Reticle patterns to be inspected can be arranged in various ways. For example, patterns for two chips can be provided on one reticle (two-chip arrangement) as shown in FIGS. 16(a) and 16(c), or a pattern for one chip can be provided on one reticle (one-chip arrangement) as shown in FIG. 16(b). Each of A, B, C, F and G in FIGS. 16(a)-16(c) represents a transfer-pattern forming portion for one chip. In the case of the two-chip arrangement, chips having the same patterns are in many cases repeated in order to increase the production yield. Respective chips are isolated by a non-pattern region LS, having a width equal to or less than 1 mm, called a scribing line.
The conventional inspection method by laser-beam scanning as shown in FIG. 15 is based on a principle in which basically, each surface to be inspected is scanned with one light beam irrespective of chip arrangement (one-chip arrangement or a plural-chip arrangement) on a reticle, and if a foreign particle is present, only light scattered by the foreign particle is detected by spatially separating as much as possible the scattered light from the light diffracted by a circuit pattern.
FIG. 17 shows a second example of the prior art, which is an apparatus for comparatively inspecting a plurality of chips arranged on a wafer. In FIG. 17, there are shown illuminating systems 20. Objective lenses 22 image respective chip patterns onto position sensors 23. A plurality of chips having the same pattern are repeatedly formed on the wafer. The optical axes of the objective lenses 22 are set onto the same patterns within two of the plurality of chips. The principle of the apparatus resides in comparing respective picture elements of the two position sensors 23 by a comparator 24, which outputs a signal only if an abnormality (a defect or a foreign particle) is present in any of the chips.
Although the first conventional apparatus (shown in FIG. 15) has the advantage that inspection time is short because of laser-beam scanning, the apparatus is required to have an ability to detect a foreign particle or the like with higher precision due to both a reduction in the size of a particle to be inspected in accordance with finer circuit patterns and an increase in pattern noise caused thereby.
Although the second conventional apparatus (shown in FIG. 17) has a high ability to detect a particle, which may be further increased by increasing the magnification of the objective lens, the apparatus has a narrow field of view which will be further reduced by increasing the magnification of the objective lens. Hence, in order to inspect the entire range to be inspected, the size of the detection unit must be increased by increasing the number of picture elements, or the mask must be frequently moved. A larger detection unit will result in a larger apparatus. On the other hand, when the mask must be moved frequently, the entire inspection time is longer than that of the laser-beam scanning method by one order of magnitude. For example, while it requires less than ten minutes to inspect a mask having a size of 100 mm square by the laser-beam scanning method, it requires more than 100 minutes to inspect the same mask by the image comparison method.